(1) Field of Invention
This invention relates generally to amplifiers and more particularly to class B push-pull transistor power amplifiers suitable for use with high power loads.
(2) Summary of the Prior Art
Class B transistor power amplifiers are known to be efficient, relatively distortion free devices capable of providing power outputs substantially greater than single transistor circuits. For this reason a great deal of work has been done in the past toward the elimination of the phenomenon of crossover distortion and the compensation of alterations in the bias current arising from the unavoidable dissymmetry of transistors and the inherent sensitivity of transistors to changes in ambient temperature. In recent years, however, the demands for higher and higher output power capability have placed severe strains upon the abilities of this type of circuit.
Consider for example the class B transistor power amplifier shown by Myer in U.S. Pat. No. 3,537,023. In that circuit Myer has demonstrated that in a class B transistor power amplifier having a driver stage and a power stage it is possible by maintaining a substantially constant bias voltage between the base electrodes of the power stage transistors via the driver stage and by providing both alternating and direct current feedback to virtually eliminate crossover distortion and to center bias of the driver stage. As power outputs increase, however, the output transistors of Myer will tend to heat and as they heat the base-emitter junction resistance will decrease thereby increasing the emitter to collector leakage current. This situation leads to thermal runaway and the eventual destruction of the output transistors.
To compensate for this regenerative heating it is common in the art to use large heat sinks around the output transistors and in some cases to use fans to dissipate the destructive heat built up. Heat sinks and fans are costly additions to circuits of the above type in terms of weight, space, and circuit complexity, not to mention the added financial burden the addition of such devices entails. Consequently, some workers in the art have attempted to compensate for this regenerative heating by the use of thermally responsive circuit elements in the input section of the power stage in order to reduce the current bias on the transistors of that stage.
Thus, in one type of circuit a thermistor is connected between the bases of the output transistors which are also resistively connected to the collectors of the drive transistors of their respective branches. In this way a current splitting configuration is created wherein by choosing a thermistor whose resistance varies with temperature proportionally with the decrease base-emitter junction resistance in the output transistors, with increasing temperature, it is possible to vary the current bias on the output transistors in such a way that the regenerative effects of increased temperature are removed. Such circuits however suffer from the unreliability of thermistors which in some cases do not vary in resistance below a certain specified temperature and which may in many cases be expected to deviate from the desired rate of resistance change with temperature. Accordingly, while circuits of this type allow savings over those using fans and large heat sinks, they are undependable in the sense that an over compensated current bias will adversely effect output and an under compensated bias will eventually lead to runaway. Further, the resistive connection of the driver stage to the power stage will cause irritating signal losses and the thermistor itself may introduce noise.
A second type of compensating means for such circuits consists of an amplifier connected in common emitter configuration with the output point of the amplifier. In this circuit the respective bases of the power stage transistors are resistively connected to the collectors of the driver stage transistors and the bases of the compensating means are current biased respectively by the emitters of the respective power stage output transistors. In this way a current splitting configuration is again created in the input to the power stage which will alter the current bias on the bases of the output power stage transistors as the base-emitter junction resistance of these transistors decreases with increasing temperature. Again, however, signal loses are inherent in the current splitting configuration and noise is introduced into the circuit. Further, the very temperature sensitivity of transistors, which the compensation means is attempting to correct for in the power stage, will effect the compensation amplifier. In such a case the over compensated current bias to the bases of the output power stage transistors will adversely effect the output in the same manner as the above described over compensated thermistor method.